1. Field of the Invention
The present invention relates to a method of superfinishing the cylindrical surface of parts intended to be mounted in circular guides and to be moved in translation along their axis relative to said guides.
2. Description of the Prior Art
Many machine parts move in translation, such parts being mounted with their cylindrical surfaces in circular guides (translation bearings). One illustrative example of a part of this kind is an automobile vehicle steering rack.
The cylindrical surfaces of such parts are generally straightened before they are subjected to a superfinishing operation, for example lapping, if necessary. All such machining operations carried out with the parts rotating form on the parts more or less fine circular or helicoidal scratches or striations, the orientation of the scratches or striations being conditioned by the relative movement of the part and the tool during machining.
Such striations or scratches, even if they are very fine, cause unwanted noise, friction and vibration when the parts move in axial translation relative to their guides.
To remedy this drawback, i.e. to eliminate the circular or helicoidal striations or scratches, it has already been proposed to submit the cylindrical surface of such parts to xe2x80x9clongitudinal superfinishingxe2x80x9d which consists of machining it with the lateral surface of an abrasive wheel driven in rotation about its axis, which is oriented perpendicularly to the axis of the part, and by moving the wheel or the part in translation along the axis of the part. This xe2x80x9clongitudinal superfinishingxe2x80x9d eliminates the circular or helicoidal striations or scratches resulting from conventional straightening or superfinishing at the cost of producing longitudinal striations or scratches which, because they are parallel to the axis of the part, cause much less vibration, friction and noise when the part is afterwards moved in axial translation relative to its guide.
In the longitudinal superfinishing method used until now, a plurality of successive passes are performed on the cylindrical surface to be machined, each pass machining a segment of the cylindrical surface of the part, which is not rotated and is offset angularly between two successive passes until the whole of the cylindrical surface has been machined by means of a plurality of successive axial passes.
The prior art method is relatively slow and difficult to automate.
Also, longitudinal superfinishing by the prior art method is effected by direct contact between a flexible abrasive wheel and the surface to be machined, which leads to relatively fast wear and soiling of the abrasive wheel.
The present invention is aimed at a method of xe2x80x9clongitudinal superfinishingxe2x80x9d of the cylindrical surface of parts which enables the parts to be machined faster and which can be extensively automated.
The invention is also aimed at a xe2x80x9clongitudinal superfinishingxe2x80x9d method which reduces the propensity to wear and soiling of the prior art method.
The method in accordance with the invention of superfinishing the cylindrical surface of a part which is intended to be mounted in circular guides and to be moved in translation along its axis relative to the guides consists of placing an abrasive applicator wheel so that its axis is perpendicular to the axis of the part and its lateral surface is pressed against the cylindrical surface, driving the wheel in rotation about its axis and moving the part and/or the wheel so that the cylindrical surface of the part is abrasively machined longitudinally in its entirety, characterized in that the wheel and/or the part is simultaneously moved in translation along the axis of the part and rotated about the axis at speeds such that the whole of the cylindrical surface is abrasively machined longitudinally along a helical trace whose successive turns overlap.
The cylindrical surface of a part can therefore be machined all at once in a single pass, and thus in an operation that is easy to automate.
From the point of view of implementing the method, it is simpler to rotate the part about its axis and to move the abrasive applicator wheel in translation along the axis of the part.
In accordance with the invention, the abrasive is preferably applied by a wheel at least the outside lateral surface of which is flexible.
The outside lateral surface of the wheel preferably includes a circumferential groove having a circular arc profile whose radius is substantially equal to the radius of the cylindrical surface of the part to be machined.
In a preferred embodiment of the invention the abrasive is applied by passing an abrasive strip over the lateral surface of the wheel and the strip is caused to perform a continuous forward movement during machining by rotating the wheel.
Although an abrasive strip of finite length can be used in the context of the invention, paid out from a pay-out spool and rewound onto a take-up spool, an endless abrasive strip is advantageously used.
The wheel is preferably pressed with a controlled constant force against the cylindrical surface to be machined.
The machining method according to the invention and one example of a machine for implementing the method are described in more detail hereinafter with reference to the accompanying diagrammatic drawings.